1. Field of the Invention
The present invention relates to methods for improving the debinding rates of ceramic and metal forms which use a polymeric binder system for powder injection molding.
2. Background of the Prior Art
Metal Injection Molding (MIM) and its counterpart for Ceramic Injection Molding (CIM) are structural part fabrication technologies that combine the design flexibility and high volume, relative low cost processing of plastic molding with the material properties of ceramics and metals. MIM and CIM are near net shape processes that require little or no post processing. These two technologies are sometimes referred to as Powder Injection Molding (PIM). See, for example, U.S. Pat. Nos. 4,624,812; 5,080,846; 5,155,158; and 5,043,118.
PIM adopts the low cost, precision molding process developed for thermoplastic polymers and adapts it to the precision shaping of structural metals, alloys, ceramics, cemented compositions and microstructures such as ceramic reinforced intermetallic matrix composites.
Certain processes are currently used instead of MIM. These include metal working (machining), investment casting (precision casting), and powder metallurgy (press and sinter).
Machining is generally favored when the number of parts to be made are small. The flexibility and inexpensive set-up cost of machining gives it an economic advantage at low production rates. However, as the required production volume increases, the increase in labor cost causes other processes to be favored.
Powder forging, press and sinter are generally favored in the production of components at low cost and high production rates. However, the level of detail and complexity that can be designed into these parts is limited. Simple geometries like transmission gears can be produced at very high production rates and low cost using this process. Without special post processing these parts tend to be porous and have a lower density.
Investment casting can generate a wide variety of cast part sizes. Large parts are more favorably produced using investment casting because of its raw material cost advantage. Smaller parts in general require more post-finishing which offsets the initial raw material advantage of investment casting. Part design is in general more limited with investment casting verses MIM.
MIM processing is generally most beneficial in high performance situations, for the production of components with complicated designs and where high productivity is desired. CIM is often the only process available to produce the desired ceramic components, since ceramics can not be melt processed.
In the debinding processes available for PIM, thermal debinding, or pyrolysis, is can be used for very small parts. The green part is heated in a closely controlled oven up to a temperature just below the softening point of the binder. The heating rate must be relatively slow to prevent thermal stresses and/or "melting/softening" of the parts. The binders designed for pyrolysis are often a combination of waxes, organic acids, and polyolefin polymers. Often there are several temperatures that the parts are held at to pyrolyize a given component of the binder.
While the capital costs are relatively low for a pyrolysis oven, the debinding rate and process tends to be very slow and there is part distortion. Control of the oven temperature must be uniform to obtain even debinding of the parts, and to avoid distortion defects caused by softening/melting of the parts. For example, a 1/4" thick part can take days to properly debind by pyrolysis. In addition, there are thickness limitations with pyrolysis also. Thick parts debind very slowly. At part thicknesses much above 3/8" the debinding rate drops toward zero due to capillary condensation.
Solvent debinding is an alternative process that improves the debinding rate verses pyrolysis. The parts are immersed in liquid or vapor of an extracting solvent. The solvent accelerates the removal of binder from the parts and helps open-up porosity in the part. Solvent debinding still requires that the residual binder and solvent must be removed from the part thermally.
The advantage of solvent debinding is that it increases the debinding rate of the parts over pyrolysis. However, the disadvantages of the process include solvent disposal. An added concern is that many of today's solvents contain chlorine and are being phased out because of the concerns with the ozone layer and the Montreal protocol.
In addition, there is part distortion due to excessive softening of the green part. In conventional debinding processes the binder is softened by heating or solvent action. Allowing the part to become too soft results in distortion or "slumping". Generally, the closer the part is to the slumping point, the faster the debinding rate. Hence, there is a compromise between debinding rate and dimensional stability. This also means that the uniformity and control of the temperature within the debinding process becomes very critical.
Further, the sintering time must also be increased to remove the residual binder and solvent remaining in the part after the process.
With solvent debinding the debinding rate decreases with thicker parts. The practical thickness limit generally falls between 3/8" and 3/4" depending on the part configuration and the specific binder system being used.
The most recently developed process of debinding is catalytic debinding. (See U.S. Pat. No. 5,073,319). In catalytic debinding a catalyst is used to break the binder into small volatile molecules. These molecules have a higher vapor pressure than the binder fragments generated in other debinding processes (pyrolysis or solvent debinding) and diffuse more rapidly out of the part.
The catalyst must be present to promote the debinding. This promotes a very uniform and rapid debinding from the exterior surface into the center of the part.
Catalytic debinding is faster, with debinding rates up to 40 times that of other techniques such as pyrolysis or solvent debinding. There is no thickness limit with catalytic debinding. The small molecules generated by the catalytic process have a high vapor pressure. This greatly minimizes the potential for capillary condensation and allows thick part sections to be debound. For example, thicknesses over 1" have been successfully processed.
As with pyrolysis and solvent debinding the debinding rate does decrease as the component or part thickness is increased. This decrease in the debinding rate has been attributed to capillary condensation and to a diffusion limited process. It has been therefore thought that a high purge gas flow rate through the debinding oven is needed to optimize the debinding rate.
The debound part is sintered at high temperature. At about half the melting temperature of the material, the powdered metals or ceramic powders coalesce together to form the final non-porous part. Sintering can be performed under inert, or reducing atmospheres, or under vacuum.
The PIM fabrication step that has been the greatest hindrance to wider application of MIM is debinding. Debinding has had several problems associated with it. While debinding via catalytic debinding is much faster than pyrolysis or solvent debinding, debinding is still a relatively slow process with debinding times generally of between about 3 to 48 hours for relatively thin (.about.3 mm) MIM parts.
While the debinding time for thick parts is often the practical limit, there is also a technical limit for debinding PIM parts usually explained as capillary condensation. This phenomena is believed to cause the debinding rate to drop toward zero for thicker parts, thus further lengthening the process.
In view of the limitations of long debinding times, there is a need for Powder Injection Molding process which provides a reduced debinding rate but still provides a high density, non-porous molded product.